1. Field of the Invention
The present invention relates to a method for manufacturing lead frame having fine leads to be connected electrically to a semiconductor element and particularly to a method for manufacturing lead frame where at least those to be connected electrically to a semiconductor element among the fine leads provided thereto have the structure of two or more layers.
2. Description of Related Art
A lead frame of this type has been manufactured to have the fine lead to be connected electrically with a semiconductor element (chip) which includes a contact area having the layer structure composed of copper (Cu) layer and gold (Au) layer for the contact with the semiconductor element. This lead frame has been manufactured by plating the fine leads of copper with gold or by forming gold bump (projected connecting electrode) to the copper fine lead.
In the method for plating the copper fine lead with gold, after a copper foil 42 is bonded to a polyimide tape 41 with a bonding agent as shown in FIG. 8A, this copper foil 42 is selectively removed by the etching process to form a fine lead 43 as shown in FIG. 8B. Thereafter, the surface of fine lead 43 is plated with gold layer 44 as shown in FIG. 8C.
Meanwhile, in the method for forming gold bump to a copper fine lead, after a fine lead 54 is formed, as shown in FIG. 9A, by plating, with copper, the surface of a copper plate 53 having formed aluminum (Al) film 51 and copper film 52 on the surface thereof, the fine lead 54 is covered with an insulated film 55 and the copper plate 53 is selectively removed by the etching process with the aluminum film 51 used as the etching stop layer as shown in FIG. 9B. Next, the aluminum film 51 and copper film 52 are selectively removed by the etching process as shown in FIG. 9C and thereafter a bump 56 is formed by evaporating gold at the end part of the fine lead 54 as shown in FIG. 9D.
However, the method of plating the fine lead with gold explained above forms a fine lead 43 by etching the copper foil 42. Accordingly, a cross-sectional area of the fine lead 43 becomes small due to generation of side etching and strength of fine lead 43 becomes remarkably weak. Therefore, this method has a problem that the fine lead 43 is easily bent at the time of forming the gold layer 44 through the plating of gold.
On the other hand, the method for forming gold bump onto the fine lead has a problem that a longer time is required for evaporation and the manufacturing process is not continuous resulting in bad manufacturing efficiency because the gold bump 56 is formed by the evaporation method. Moreover, this method also has a problem that manufacturing cost becomes high because a large number of jigs are required, in addition to the problem that it is difficult to keep the thickness of bump 56 to 3 .mu.m or more and to improve the quality because accuracy of film thickness is low.